1. Field of the Invention
The present disclosure relates to an image forming apparatus for controlling a speed of a drive system for conveying a recording material.
2. Description of the Related Art
Among electrophotographic image forming apparatuses, there is an apparatus having a configuration in which a toner image formed with toner according to image data is borne on an intermediate transfer member, and the toner image on the intermediate transfer member is conveyed to a transfer unit by driving the intermediate transfer member. In this configuration, a recording material such as a sheet is supplied to be brought into contact with the toner image on the intermediate transfer member at timing when the toner image on the intermediate transfer member reaches the transfer unit. In the transfer unit, a voltage difference is generated between the intermediate transfer member and the recording material, so that the toner image on the intermediate transfer member is transferred to the recording material. The recording material with the toner image transferred by the transfer unit is conveyed to a fixing unit. Then, the toner image on the recording material is fixed by heat and pressure of the fixing unit and discharged from the image forming apparatus.
In the image forming apparatus having such a configuration, a pair of registration rollers is disposed to stop the recording material in a predetermined position before the toner image on the intermediate transfer member reaches the transfer unit, and supply the recording material at timing when the toner image on the intermediate transfer member reaches the transfer unit.
Herein, in an area near the transfer unit in a direction to which the recording material is conveyed, a force causing the toner on the intermediate transfer member to move to the recording material is generated by influence of a transfer electric field based on the potential difference between the intermediate transfer member and the recording material. Consequently, even near the transfer unit, the toner on the intermediate transfer member is transferred to the recording material. However, the force causing the toner to move to a recording material side continues to decrease near the transfer unit while the toner on the intermediate transfer member is moving to the recording material from the intermediate transfer member. Such a decrease in the force causes the toner to adhere to an area on the recording material, the area being to which toner is not originally supposed to adhere. This becomes more noticeable as a gap between the recording material and the intermediate transfer member becomes wider near an upstream side of the transfer unit in a conveyance direction of the recording material, thereby causing deterioration in quality of an image formed by the image forming apparatus.
Japanese Patent Application Laid-Open No. 8-240954 discusses the transfer of a toner image on an intermediate transfer member to a recording material in a state that the recording material being conveyed to a transfer unit is slightly distorted toward the side of the intermediate transfer member. Specifically, a speed of conveying the recording material by registration rollers is set slightly higher than that by the intermediate transfer member in the transfer unit, so that a gap between the recording material and the intermediate transfer member is narrowed on an upstream side relative to the transfer unit in the conveyance direction of the recording material.
However, since an outer diameter of the registration roller changes due to a change in humidity and temperature of environment where an image forming apparatus is placed, there are cases where a speed of conveying the recording material by the registration rollers fails to reach a target speed. For example, when an outer diameter of the registration roller increases, a speed of conveying the recording material by the registration roller becomes higher than a target speed. Thus, if a conveyance speed of the recording material becomes higher than a speed of the intermediate transfer member to be driven, an image to be formed on the recording material has an area having a higher density than a target density.
Such a problem occurs when the speed of conveying the recording material by the registration roller exceeds the target speed, and becomes higher than the driven speed of the intermediate transfer member. This higher speed causes an increase in a distortion amount of the recording material, thereby damaging a toner image on the intermediate transfer member by the recording material. In other words, when a distortion amount of a recording material becomes larger than a predetermined amount while the recording material is passing a transfer unit, a surface of the recording material is repeatedly brought into contact with and separated from a toner image on an intermediate transfer member. The toner image on the intermediate transfer member can be damaged by being brought into contact with the surface of the recording material for many times.
Consequently, when the toner image damaged by being brought into contact with the recording material is transferred to the recording material, a print product having a low quality image is formed.